Removed the thermostat from the position near the exhaust manifold. Reassembled the engine with the thermostat on top of the head near the water pump - the other place it appears in the parts manual and the only place it appears in my service manual.

Well, the coolant in the heat exchanger now heats to 160 degrees - before it was room temperature - indicating (at least to me) that the water pump is working and the new thermostat in the head position is working.

The ran at normal temperature - just below 190 - for 20 minutes at 1600 RPM.

When I increased the speed t 3200 RPM the temperature rose rapidly to 235 degrees when the overheat alarm triggered.

At that point, I measured the zinc cap on the heat exchanger at 160 - seems about normal - and the raw water outlet at about 110, also seems normal. The domestic hot water inlet was 135 - low, but it had cooled.

I do not see the leak - and can not get the new 20 PSI cap off until it cools.

Question: Any idea where I should look for the leak?????

Question: After a run, what is the temperature at the heat exchanger zinc? Thank you

Well, once you get the new cap on I would pressure the system to 20 PSI and see if it holds pressure. If it does, IMO, that would indicate a blockage rather than a leak. My next step to confirm that would be to remove the thermostat and reassemble without it and see what kind of temperature increase you get. When my thermostat dislodged I could not get more than 145 degrees on my temperature gauge. Keep in mind mine is a 180, so the temperature may be slightly higher.

“What we call reality is merely an ILLUSION we create for learning and enjoyment….”

The mechanics finished up - water is now circulating thru the heat exchanger when the engine is hot.

I took the boat for a test this morning - ran fine up to 2300 rpm than as I accelerated to 3900 rpm, the temperature suddenly went from 205 to 230 degrees - indicating to me there was a loss of coolant incident.

When I checked more carefully, the heat exchanger cap was only partially on, the overflow bottle was full, and water was everywhere where it had sprayed out of the overflow bottle.

I took off the cap (about 45 minutes), put 2 quarts of water in the heat exchanger and put the cap back on.

The engine ran OK but still over heated after 5 minutes or so.

This may be another loss of cooling incident or it may be air entrapped in the water lines.

The local Yanmar folks say I have to burp the engine by opening a burping valve?????????

The mechanics removed the fitting between the exhaust manifold and the hot water return line. It is sealed with two "O" rings that appeared crushed. They replaced them.

On observation during the sea trial after the above "O" ring replacement, we observed the raw water flow thru the sea chest STOPED at about 3000 RPM. Wow. On careful inspection, the mechanic found still another busted hose clamp - the 4th we have found - this one on the hose from the sea chest to the wash down pump. The mechanic also found the "O" ring in the water wash-down pump inlet fitting was mis-seated.

We agreed there was no real reason to retain the wash-down pump until this overheat is fixed, so the mechanic moved the flush hose bib fitting from the "T" on the wash-down pump to the sea chest. When we tested after this change, the water no longer stopped flowing at 3000 RPM, but the overheat continued, although it took much longer to take place.

Next, I cleaned the inlet screen on the raw water inlet - it was partially clogged with growth even tough the bottom was painted and the screen cleaned in May.

The overheat continued - but took 10+ minutes to overheat.

Tomorrow morning, I will pry open the inlet screen and clean it more completely.

Each time, we find a new problem- correct the problem - and the overheat symptoms change slightly.

Now, the heat exchanger is rising to about 200 degrees when the overheat occurs - so it is unlikely the problem is in the coolant circuit.

I need a great idea.

A lesson learned again: If you have any original Ranger-supplied hose clamps inspect them carefully or change them for safer clamps. I have no way of knowing if Ranger installed a bad batch or mis-installed the clamps, or they were abused by a former owner . . . but, they don't seem to do their job any longer.